Clockwise from top left: A rare male king cheetah, a lion, a pangolin
and a orangutan, all members of species that have experienced sharp declines in
recent decades.CreditClockwise from top left: John Wessels/Agence France-Presse
— Getty Images, Ben Curtis/Associated Press, Roslan Rahman/Agence France-Presse
— Getty Images, Ulet Ifansasti for The New York Times

From the common barn swallow to the exotic giraffe, thousands of animal
species are in precipitous decline, a sign that an irreversible era of mass
extinction is underway, new research finds.

Gerardo Ceballos, a researcher at the Universidad Nacional Autónoma de
México in Mexico City, acknowledged that the study is written in unusually
alarming tones for an academic research paper. “It wouldn’t be ethical right
now not to speak in this strong language to call attention to the severity of
the problem,” he said.

Dr. Ceballos emphasized that he and his co-authors, Paul R. Ehrlich and Rodolfo Dirzo, both professors at Stanford University, are not
alarmists, but are using scientific data to back up their assertions that
significant population decline and possible mass extinction of species all over
the world may be imminent, and that both have been underestimated by many other
scientists.

They found that about 30 percent of all land vertebrates — mammals,
birds, reptiles and amphibians — are experiencing declines and local population
losses. In most parts of the world, mammal populations are losing 70 percent of
their members because of habitat loss.

In particular, they cite cheetahs, which have declined to around 7,000
members; Borneo and Sumatran orangutans, of which fewer than 5,000 remain;
populations of African lions, which have declined by 43 percent since 1993;
pangolins, which have been “decimated”; and giraffes, whose four species now number under 100,000 members.

The study defines populations as the number of individuals in a given
species in a 10,000-square-kilometer unit of habitat, known as a quadrat.

Jonathan Losos, a biology professor at Harvard, said that he was not
aware of other papers that have used this method, but that it was “a reasonable
first pass” at estimating the extent of species decline and population loss.

Dr. Losos also noted that giving precise estimates of wildlife
populations was difficult, in part because scientists do not always agree on
what defines a population, which makes the question inherently subjective.

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Despite those issues, Dr. Losos said, “I think it’s a very important and
troubling paper that documents that the problems we have with biodiversity are
much greater than commonly thought.”

The authors of the paper suggest that previous estimates of global
extinction rates have been too low, in part because scientists have been too
focused on complete extinction of a species. Two vertebrate species are
estimated to go extinct every year, which the authors wrote “does not generate
enough public concern,” and lends the impression that many species are not
severely threatened, or that mass extinction is a distant catastrophe.

Conservatively, scientists estimate that 200 species have gone extinct
in the past 100 years; the “normal” extinction rate over the past two million
years has been that two species go extinct every 100 years because of
evolutionary and other factors.

Rather than extinctions, the paper looks at how populations are doing:
the disappearance of entire populations, and the decrease of the number of
individuals within a population. Over all, they found this phenomenon is
occurring globally, but that tropical regions, which have the greatest
biodiversity, are experiencing the greatest loss in numbers, and that temperate
regions are seeing higher proportions of population loss. Dr. Ehrlich, who rose
to prominence in the 1960s after he wrote “The Population Bomb,” a book that predicted the imminent collapse of humanity because of
overpopulation, said he saw a similar phenomenon in the animal world as a
result of human activity.

“There is only one overall solution, and that is to reduce the scale of
the human enterprise,” he said. “Population growth and increasing consumption
among the rich is driving it.”

He and Dr. Ceballos said that habitat destruction — deforestation for
agriculture, for example — and pollution were the primary culprits, but that
climate change exacerbates both problems. Accelerating deforestation and rising
carbon pollution are likely to make climate change worse, which could have
disastrous consequences for the ability of many species to survive on earth.

Dr. Ceballos struck a slightly more hopeful tone, adding that some
species have been able to rebound when some of these pressures are taken away.

Dr. Ehrlich, however, continued to sound the alarm. “We’re toxifying the
entire planet,” he said.

When asked about the clear advocacy position the paper has taken, a
rarity in scientific literature, he said, “Scientists don’t give up their
responsibility as citizens to say what they think about the data that they’re
gathering.”